1. Field of the Invention
The present invention relates to a semiconductor device using an oxide semiconductor.
In this specification, the semiconductor device refers to any device that can operate utilizing semiconductor characteristics. A transistor described in this specification is the semiconductor device, and an electrooptic device, a semiconductor circuit, and an electronic device each including the transistor are all included in the category of the semiconductor device.
2. Description of the Related Art
Transistors used for most flat panel displays typified by a liquid crystal display device or a light-emitting display device are formed using a silicon semiconductor such as amorphous silicon, single crystal silicon, or polycrystalline silicon provided over a glass substrate. Further, such a transistor employing such a silicon semiconductor is used in integrated circuits (ICs) and the like.
Attention has been drawn to a technique in which, instead of the above silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used for transistors. In this specification, such a metal oxide exhibiting semiconductor characteristics is also referred to as an oxide semiconductor. For example, a technique is disclosed in which a transistor is manufactured using a Zn—O-based oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor for application as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).
Meanwhile, it has been pointed out that hydrogen behaves as a source of carriers in an oxide semiconductor. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor in forming the oxide semiconductor. Further, a shift of a threshold voltage has been suppressed by reducing the amount of hydrogen contained in not only the oxide semiconductor but also a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).